By Paul McLearyPaul McLeary is Foreign Policy’s senior reporter covering the U.S. Defense Department and national security issues. He joined the Washington office in 2015 after working for Defense News, where he was also on the Pentagon beat, and covered stories relating to Pentagon spending and the defense industry. While there, and in a previous incarnation as a New York-based reporter, Paul embedded with U.S. Army and Marine Corps units in Iraq and Afghanistan to cover ground combat operations, where he got inside a secretive drone program being run out of Bagram air base. He has also traveled with the U.S. Navy, covered NATO meetings in Europe with the chairman of the Joint Chiefs of Staff, and stalked major international arms shows in Paris and London.

President Barack Obama’s top military advisor told senators Thursday that the U.S-backed Syrian Democratic Forces are the only rebel group capable of taking the Islamic State bastion of Raqqa. But there’s no solid plan for holding the city once they do.

The 30,000-strong SDF is dominated by about 15,000 hardened Kurdish fighters that Washington has long backed in the struggle against Islamic State. But the Americans are trying to balance supporting their most capable ally on the ground with intense opposition from another ally — Turkey — which views the Kurds as terrorists.

Dunford called the group “the most effective force that we have right now and the force that we need to go into Raqqa,” but admitted that given its mainly Kurdish makeup, the group “is not intended to hold Raqqa” once it falls. That role, potentially, could be filled by the roughly 14,000 Arabs in the SDF, which may make up “part of the holding force,” he told the Senate Armed Services Committee. When asked at the hearing if the Pentagon had a plan to take and hold the city, Dunford admitted, “we have a plan but it is not resourced.”

On Wednesday, the general lauded the work of U.S.-backed rebels, particularly the effort to boost the number of Arabs in the SDF. “Last year at this time we probably had a few hundred vetted Syrian opposition forces on the ground we were supporting, so there has been significant growth,” he said.

American officials have for months touted the SDF as the only group capable of taking Raqqa away from the Islamic State. But with Turkish concerns mounting over Kurdish gains on the battlefield and U.S. support, Washington has made more of an effort to build up the capabilities of the Syrian Arab rebel groups aligned with the Kurds.

Tensions with Turkey, which last month deployed 1,000 Arab rebels along with Turkish special forces and heavy armor into northern Syria, remain high. The Turkish incursion over the border — which initially took Washington by surprise — was quickly supported by American commandos on the ground and warplanes overhead. Ankara considers the Kurdish fighters in Syria to be part of the Kurdish PKK rebel group fighting to gain an autonomous region in southeast Turkey, and has bristled at U.S. military support for the Kurds. Turkey fears a Kurdish state forming on its southern border that could inflame its own Kurdish minority. One of the invasion’s explicit aims was to push the Kurds away from Turkey’s border with Syria.

The Kurds have also objected to U.S. support of the Turkish incursion into Syria, as it forced Kurdish troops to leave the city of Manbij — which they had taken from Islamic State — and move back to Kurdish-held areas east of the Euphrates river.

A team of American special operations forces accompanying the Turks last week were pulled into the middle of these tensions, when some Turkish-backed fighters accused the Americans of siding with the Kurds over Arabs. Turkish officials moved the rebels who had complained to another area of the battlefield, and the Americans are still on the ground in northern Syria, according to U.S. Defense officials.

“We’re working very closely with our Turkish allies to come up with the right approach to make sure we can conduct effective and decisive operations in Raqqa with the Syrian Democratic Forces and still allay the Turkish concerns about the Kurds long term political prospects,” Dunford said.